The measurement of torque values, as for example, on a shaft driven by a valve actuator, is an expensive, time consuming and possibly inaccurate process which is dependent on many factors such as test stand alignment and geometry, welding and materials of the test stand and unknown friction of the testing system. Friction measurements may have to be done on a completely separate test stand which is a costly and time consuming inefficiency.
A conventional torque measurement system and method involves the use of a fixed, immovable test stand which may be a spool subtended by upper and lower flanges with a shaft welded to the lower flange and extending up through the spool and upper flange. The device to be tested is bolted to the upper flange with the upper end of the shaft being inserted into the device. Strain gauges, which produce electrical signals in proportion to shear forces acting thereon, are diametrically mounted on the shaft and strain gauges are also mounted on the periphery of the spool. In order to mount the gauges on the shaft, a window must be cut through the side of the spool.
Under theoretically ideal conditions the torque developed along the shaft by the device under test would be fully transferred and sensed by the strain gauges and the data could be interpreted using well-known stress-strain equations. However, the use of conventional welding techniques, spool materials, and the complicated geometry of the spool itself disrupts the stress uniformity and prevents accurate mathematical strain calculations. Therefore, interpretation of the strain gauge readings is difficult and inaccurate. Alignment of the device when it is bolted to the spool will also affect the stress distribution throughout the test stand. Calibration of the test stand can be done but such a process is costly and complicated. As a result, errors as high as 10-15% can be expected when measuring torque by such conventional methods.
It is appreciated that the friction characteristics of an actuator when energized and under a load are different from the friction characteristics of the same actuator when it is not so energized. To accurately determine the operating capabilities of an actuator, therefore, it is necessary to ascertain the friction characteristics of the actuator when it is energized and under a load. Inasmuch as the test shaft of the prior art test stand is fixed and the output shaft of the actuator cannot rotate when coupled thereto, the friction in the test system when the actuator is under a load cannot readily be determined.
It is, therefore, apparent that the state of the prior art is such that the need exists for a comprehensive method and apparatus which can accurately and economically measure the torque produced by a device and the frictional force of that same device when it is under a load condition, independent of test stand structural or material characteristics.